This invention relates generally to structural composites and, more particularly, to structural composites formed from a fiber reinforced resinous material that is fused to a cellular core material, to provide a light weight, monolithic composite panel of exceptionally high impact strength and load bearing capabilities. The invention also relates to the process for constructing such composites.
Typically, resinous composites are formed by curing a mass of reinforcing fibers coated with one of several resinous matrix materials, such as polyester or epoxy resins. The reinforcing fibers and resinous materials are combined in such a way as to assimilate the materials as much as possible. That is, the fiber material and the resin matrix material are blended or intermixed into a substantially homogeneous mass such that when the resin matrix material is cured or hardened, the intermixed materials are integrally bonded or fused together, hence producing a reinforced fiber material. When formed in the shape of a usable part or structure and allowed to cure into an integrally bonded or fused mass, such reinforced fiber material is referred to as a composite. Composite parts of this type are commonly formed using known technology.
There are a variety of resinous matrix materials, as well as fibers, that have been used in the production of fiber reinforced composites, the selection of the various possible materials generally being dictated by the desired end use, or application, of a given composite. Typically, glass, carbon, boron, metal or polymeric fibers, such as polyester or polyamide (nylon) fibers, are used as the reinforcing fibers. These fibers may be used in a variety of configurations, for example, in the form of rovings, tapes, mats, woven and non-woven webs, tapes, or the like. The resins which are used as the matrix materials normally comprise various heat hardenable epoxies, phenolics, nylons, polyesters, polyurethanes or polysulfides. The materials are well known and are readily available in commerce.
Reinforced fiber composites are well known materials suitable for use in a wide variety of industrial, commercial and structural applications. Generally, such composites are relatively strong and having unusual load bearing strength in relation to their weight, particularly when formed in the shape of a structural member such as a channel ("[") or I-beam.
There are a number of known non-metallic cored composite structures, some of which utilize end-grain, balsa-wood materials. Light weight, cored composite parts are widely used to withstand static loads as floor beds for railroad cars, shipping containers, cargo pallets and helicopter pads, or as dividing structural members such as bulkheads, doors, reefer bodies, etc. These materials are also employed for structural insulation in aircraft applications, in housing and in boating where bulk with minimum weight is required. In essence, they are characterized by their ability to withstand static loads or as structural members. Conversely, they are not characterized by their ability to withstand excessive impact loads without structural support. Examples of such structural members and composite materials are those described in U.S. Pat. Nos. 3,298,892; 3,325,037; 3,376,185; 3,487,489; 3,616,111; 3,901,993; 3,905,172; 4,209,486; 4,271,649; 4,343,846; 4,533,589; and Canadian patent 793,014.
Recently, in U.S. Pat. No. 4,801,483, there was disclosed a fiber reinforced composite which is able to successfully withstand and respond to a moving, concentrated, impacting shock of excessive weight, such as a 20 ton per square foot impact load, without some type of reinforcing support, such as the ground or roadway surface. The composite product disclosed in that patent is a light weight, high strength trench cover used to span holes or openings (supported two sides only) on surfaces carrying vehicular traffic of all types and weight, some of which may very well exceed 40 tons.
In addition to the use of reinforcing fibers and resin matrix materials, the composite product disclosed in U.S. Pat. No. 4,801,483 included a non-fiber reinforcing agent, such as end-grain, balsa-wood blocks as a core material. End-grain balsa-wood is a particularly useful core material for preparing composites parts characterized by exceptionally light weight and high load bearing strength, since it is dimensionally stable and has a cell structure which affords a combination of high rigidity, and compressive and tensile strength that is superior to other materials of comparable density. In the composite disclosed in U.S. Pat. No. 4,801,483, a combination of end-grain balsa-wood blocks and matrix resins form a core that is "bonded on all surfaces" to a reinforced fiber/resin matrix shell. The balsa-resin core is bonded to the reinforced fiber/resin matrix shell so that the shell and the core move together as one (in complete unison) under load, and form a light weight panel that not only can support excessive static loads, but that can also withstand excessive impact loads. In the patented composite product, it is the core structure, and the way in which it adheres to the surrounding reinforced fiber/resin matrix shell, that allows the panel to go from an immobile, unshocked state to a state where immediate response to a moving, irregular impact load, of as much as 20 tons per square foot, is required.
The composite product disclosed in U.S. Pat. No. 4,801,483, was developed as a safer, more economical alternative to the use of one inch thick steel plating as a cover for holes, trenches or openings in a roadway surface. The weight relationship of one (1) square foot of one inch steel plate to one (1) square foot of the patented composite plate is 6.7 to 1, whereas the deflection, under maximum point load, of approximately 0.90 inches, is the same for both the steel plate and the patented composite plate.
The patented composite is generally sold as a plate measuring 5 feet wide.times.6 feet long and weighing 182 lbs., and being rated for street and highway use. The primary advantages of the patented plates are:
1. They are non-corrosive and will not be affected by roadway chemicals, salt or water.
2. Their handling requires no special crews or equipment such as flat bed trucks, loaders, or boom trucks and the crews necessary for their operation.
3. Because of their light weight and ease of handling the plates require less time when opening and closing a hole. Based on crew size actual manhour savings could be substantial.
4. In off hours or during emergency situations the plates can be distributed quickly and easily to a problem area and installed by a repair crew.
5. Because of their light weight and design the plates are much safer to use than steel road plates for both the craftperson working on the job site and vehicular traffic moving near or over the job site.
6. The plates are much less expensive to use than steel roadway plates. A recent study conservatively estimated the handling cost of steel to be $28.64 per linear foot. The cost of the lighter weight patented plating was estimated to be $3.20 per linear foot; a difference of $2544.00 per 100 feet of plating. This study did not consider either the reduced cost of accident claims or the cost of non-productive crew time during the plating operations.
Until the development of the plate described in U.S. Pat. No. 4,801,483, there was not a light weight, load bearing product having high impact strength that was commercially available as a replacement for steel plating as a ground opening cover. However, because of its limited strength to weight characteristics and the molding process currently necessary for its production, the patented plate has a number of disadvantages that limit its utility and, therefore, its acceptibility in the market place. For example, the patented plates:
1. have a strength to weight relationship that:
(a) requires a material thickness of 2" to safely carry a 41,600 lb. axle load over a span of 4 ft.; a ratio of 1/2" to 1 ft. PA1 (b) requires a substantially increased, and disproportionate, material thickness to safely carry a 41,600 axle load over a span in excess of 4 ft. For example, to span a hole of 6 ft. the patented panel would require a thickness of approximetely 4 inches; a ratio of 3/4" to 1 ft.
2. are too thick to permit their use, on 4 ft. spans, in areas having vehicular traffic that moves at speeds in excess of 35 miles per hour.
Thus, while the composite roadway plates, disclosed in U.S. Pat. No. 4,801,483 represent a considerable advancement over the prior art, with their current weight to strength ratio the plates must be made too narrow to be used as an effective replacement for steel plating, and if made wider would be too thick to be safely used on a surface carrying vehicular traffic.